Spinal corrective systems may be used in orthopedic surgery to correct a deformity or misalignment in the spinal column, as well as stabilize and/or fix a desired relationship between adjacent vertebral bodies. Spinal corrective surgery may be used to correct a variety of defects in the spine, such as scoliosis, spondylosis, spondylolisthesis, vertebral instability, spinal stenosis, degenerated, herniated, or degenerated and herniated intervertebral discs, as well as injuries, including fractures and torn ligaments and surgical manipulations, including laminectomies.
In spinal corrective surgery, a spinal corrective device, such as a spinal rod, and bone anchors, such as screws, bolts, wires and/or hooks, may be used to stabilize the spine, repair a defect and/or bring misaligned vertebrae back into alignment and secure the aligned vertebrae in the aligned position. A standard surgical procedure for correcting spinal defects in the current state of the art involves first distracting the vertebrae, pulling the vertebra back into alignment with the spinal column, and then stabilizing the spine using posterior spinal implants consisting of anchoring devices and spinal corrective elements. An interbody fusion device may also be used to give further stability and correction of the disc height. Compression across the vertebrae may be applied across the construct to set the correct balance of forces in the region.
A spinal corrective device used in such systems is generally a relatively rigid fixation rod or plate that is coupled to a bone by attaching the element to various anchoring devices. The corrective device can extend between two bone regions to effect stabilization, positioning, reduction and/or fixation of the bones. The spinal corrective device can have a predetermined contour that has been designed according to the properties of the target implantation site and, once installed, the spinal corrective element holds the bones in a desired spatial relationship, either until desired healing or spinal fusion has occurred, or for some longer period of time.
Shape memory materials have been used in spinal corrective systems to apply corrective forces to the spine by transitioning a shape memory material in a spinal corrective device between a flexible martensitic state and a rigid austenitic state to generate corrective forces that are transferred to the spine. Shape memory materials are characterized by an ability to restore the material to a pre-selected shape of the austenitic state after plastic deformation. For example, in an austenitic state, the shape memory material is stiff, rigid and has a set, pre-selected shape. In a martensitic state, the shape memory material becomes flexible and deformable and may have any of a variety of shapes. The microstructure of the material in the martensitic state is characterized by “self-accommodating twins”, having a zigzag arrangement, which allow for deformation of the material shape by de-twinning.
Generally, shape memory materials are induced to transition between the rigid austenitic state and the flexible martensitic state by changing the temperature, pressure, stress, chemistry and/or another parameter of the shape memory material. For example, a shape memory material may be cooled to make the material flexible in a martensite state, and subsequently heated to return the material to the original shape with the austenitic structure. The temperature at which a shape memory material starts transforming to austenite is known as the “austenite start temperature.” Further heating increases the temperature of the shape memory material to induce a complete transformation to the austenitic state, setting the shape of the material in the pre-selected shape of the austenitic structure. The temperature at which a shape memory material finishes transforming to austenite is known as the “austenite finish temperature.” A shape memory material can transition to the martensitic state to allow deformation and shaping of the spinal corrective device by changing the temperature of the material below a selected temperature (i.e., cooling the material). The temperature at which a shape memory material begins transformation to the martensite state is known as the “martensite start temperature”. Further cooling decreases the temperature of the shape memory material to induce a complete transformation to the martensite state. The temperature at which a shape memory material finishes transformation to the martensite state is known as the “martensite finish temperature.”
Performing cooling of shape memory material in spinal corrective devices, such as spinal rods, from an ambient or heated state prior to implantation or following implantation can be problematic. For example, the process of handling, manipulating and implanting the spinal corrective device may create a heating effect, causing the device to transition from the martensitic state to the austenitic state before it is desirable. Shape memory corrective devices should therefore be cooled thoroughly to give maximum useful operating time.